This invention relates to battery chargers and more particularly to chargers which receive charging current from a storage battery.
One type of battery charger employs a portable DC source, such as a motor vehicle battery, which permits operation in remote locations. An application for such chargers is with portable electric tools which tend to drain the battery pack at a relatively rapid rate when operated on a substantially continuous basis. Therefore, in such applications, it is advantageous to employ removable battery packs which permit the tool to remain in service with an alternate pack while the first pack is being recharged. However, to insure continued tool availability, the battery pack must be charged at a relatively rapid rate. Furthermore, it is desirable that the charger be operable to charge a wide range of nominal battery voltages so that a variety of tools may be serviced.
In order to prevent damage to the battery pack, it is necessary to terminate the flow of charging current when the pack is fully charged. The normal method of providing such protection is to employ a thermoswitch in the battery pack which is operable to disable the charger when the temperature of the battery pack exceeds a predetermined level indicating that the battery is fully charged. Since damage may result if charging current is delivered to a fully charged battery, it is desirable to provide means for preventing the renewal of charging current when the battery pack cools.